An image forming apparatus such as a multi-function peripheral (MFP) or a printer includes a fixing device. The fixing device includes a fixing unit from which heat is transferred to a sheet having an image thereon to fix the image to the sheet, while the sheet passes through the fixing unit. The fixing unit includes, for example, a roller and an endless belt.
In general, there is a trade-off between maintaining uniformity of temperature across different positions of the fixing unit and the energy consumed by the fixing unit. When the heat capacity of the fixing unit is large, the fixing unit can be maintained uniformly at the fixing temperature even though heat is transferred to sheets as they are passed therethrough. However, when the heat capacity of the fixing unit increases, energy required to heat the fixing unit also increases. On the other hand, when the heat capacity of the fixing unit is small, the fixing unit may have a temperature difference between a region through which the sheet passes and a region through which no sheet passes. As a plurality of sheets passes through the fixing unit, the temperature of the region through which no sheet passes may become excessively high.